1. Field of the Invention
The present invention relates to an ophthalmological examining instrument and more particularly to such an instrument for examining an eye with mitigates measuring errors otherwise caused by externally scattered light such as room lamp.
2. Description of the Prior Art
For examining a human eye it is a common practice in the art to fix the eye of a person to be examined on a certain fixation object. For example, in case of an eye refractometer, a target image is optically formed in a light-shielded casing containing the functional part for measuring. The eye of a person to be examined is fixed upon the formed target image. However, it is known that this examining method has a problem. When the apparent distance from the eye to the target image is set to five meters or less, the person in practice is apt to assume a posture forwardly looking into the target image. This results in a state similar to short sightedness which is generally called as mechanical myopial. Due to the unfavourable pheonomenon, the measured eye refractive power is different from the actual refractive power of the same eye in ordinary use. For this reason it is undesirable to interpose any converging or diverging optic between a person to be examined and a fixation object in conducting an examination for myopia.
For some kinds of eye examining instruments the person to be examined is required to fix his one eye to be examined upon a target image while covering the other eye with a hand or the like. However, as is known in the art, there is some difference in refractive power conditioning between sight by two eyes and sight by a single eye. In view of this point it is preferred that the eye examination be conducted under a natural condition, that is, under the condition of sight by two eyes.
Also, it is well known that the result of an eye examination is affected by the illuminance of an image or object upon which the eye is to be fixed as well as the illuminance of the surroundings about the person to be examined. The result obtained by examining an eye in a dark room or with the eye being fixed upon an object placed in the dark is sometimes different from the result obtained by examining the same eye under ordinary brightness. For this reason it is undesirable that eye examination be conducted while the face of a person to be examined is put against a light-shielded tube of the examining instrument.
As seen from the above, when a person is compelled to look at an object under unusual conditions for conducting an eye examination, his eyes exhibit a different response from that exhibited under usual conditions. Therefore, it is desirable that an eye examination be conducted under a condition as similar to that in daily life as possible. Furthermore, many eye doctors agree that examination or measurement of a human eye should be conducted while observing the expression on the face of one whose eye is being examined or measured. They say that accuracy of the examination or measurement can be improved by doing so and this is particularly true for subjective eye examination or measurement.
In our prior application, Japanese Patent Application No. 115,647/1978 (U.S. Ser. No. 75,115, now U.S. Pat. No. 4,372,655) there is disclosed an embodiment for conducting an eye examination under the conditions with both eyes seeing and ordinary illumination. While the embodiment of the prior invention has satisfied the above requirements for eye examination, it was characterized by another problem. According to the embodiment, a beam splitter is interposed between the eye to be examined and the fixation object. A scene of the examining room is projected on a splitting surface of the beam splitter by which the examined person's attention is distracted. In addition, since the person looks at the projected scene, the direction and sight of the eye is affected by it. This is the problem involved in the embodiment of the prior invention. A similar problem may arise even when an optical and electrical filter system is provided in the measuring part. In this case, it is desired to prevent external light such as room light from entering the objective system.